In the manufacture of tubular incandescent lamps of the variety described above, it is typically necessary to seal a longitudinally supported tungsten filament within both ends of the envelope and fill the lamp with an inert gas and halogen. The function of the inert gas is to retard the vaorization of the tungsten filament during illumination thereof. The halogen is added to prolong the tungsten filament life by slowing the rate of evaporation thereof. The halogen cycle is well known, and its utilization in many lamps in today's art is well documented.
One method commonly used to produce lamps of the type described above was to press seal the supported filament structure within one end of the envelope while the other end was temporarily plugged. An inert gas was flowed into the envelope through a centrally appended exhaust tube. While flowing the inert gas through the exhaust tube, the plug was removed and the opposing open end of the envelope then press sealed, following which the exhaust tube was tipped to thus seal the envelope. Providing an atmosphere of inert gas within the envelope during sealing operations is deemed essential to prevent oxidation of the highly heated filamentary components. In order to produce lamps of the high pressure type (those wherein the final internal pressure exceeds one atmosphere, e.g., three atmospheres), it was considered necessary to immerse one end of the envelope within a coolant to lower the volume of the gas before sealing of the envelope at the central tubulation. Cooling was considered necessary in order that the relatively high internal pressure would not blow out the central "tip" area when the sealing torches were applied to accomplish tipping.
While the above described method can be utilized to produce high pressure tungsten halogen lamps, a fused tip, usually centrally located, always remains on the envelope, and in many applications is deemed quite undesirable because it may adversely affect the optical properties of the lamp. For example, when a tubular lamp having a residual fused tip is used in a photocopy machine, it has been found that the light is unevenly distributed upon the paper and, since exact and critical limitations are placed upon the distribution of light in such machines, the work was not perfectly copied. In addition, the presence of a residual tip provides physical limitations in such areas as lamp packing and subsequent placement within the apparatus (e.g., photocopy machine) utilizing the lamp.
It is believed, therefore, that a method of making a high pressure tungsten halogen lamp wherein the finished envelope does not contain a residual, protruding tip member thereon and which eliminates many of the associated disadvantages of techniques such as stated above would constitute an advancement in the art.